The Environmental Protection Agency on Monday took steps to challenge California’s decades-old right to set its own air pollution rules, setting up a showdown between the federal government and a state that has emerged as a bulwark against the Trump administration’s policies.

The E.P.A. statement was part of the agency’s widely expected decision to reconsider, and most likely roll back, Obama-era rules requiring automakers to hit ambitious emissions and mileage standards by 2025. The statement, though, was notable for the forcefulness of its language suggesting that the Trump administration would take on California’s authority to set its own rules.

Scott Pruitt, the E.P.A. administrator, signaled that he aimed to make California fall in line. The Obama administration, he said, “made assumptions about the standards that didn’t comport with reality, and set the standards too high.” California’s history of setting its own emissions rules “doesn’t mean that one state can dictate standards for the rest of the country,” Mr. Pruitt said.

A rollback of the rules, which are designed to cut back on emissions of greenhouse gases, would reverse one of the single biggest steps any government has taken to tackle climate change. California has said it will stick with the tougher, Obama-era regulations, a decision that could effectively split the United States into two auto markets: one requiring cars to be more efficient and less polluting than the other.

California has long possessed the unique authority under the 1970 Clean Air Act to write its own air pollution rules. Traditionally, a dozen other states follow California’s air pollution rules and together they represent one-third of the nation’s auto market. That puts California in an extraordinary position to stage a regulatory revolt, with much of the country’s car market in tow.

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President Trump and the E.P.A. are scaling back Obama-era rules for U.S. automakers on greenhouse gas emissions. Here’s the breakdown.Published OnMarch 30, 2017CreditCreditMark Ralston/Agence France-Presse — Getty Images

State officials indicated they would fight the Trump administration. “This is a politically motivated effort to weaken clean vehicle standards,” said Mary Nichols, California’s top air pollution regulator. California, she said, “will vigorously defend the existing clean vehicle standards.”

Xavier Becerra, the state’s attorney general, said the state was “ready to file suit.”

Adopted in 2012, the standards up for revision would have required automakers to nearly double the average fuel economy of new cars and trucks, to 54.5 miles per gallon by 2025. If fully implemented, the rules would have cut oil consumption by about 12 billion barrels over the lifetime of all the cars affected by the regulations and reduced carbon dioxide pollution by about six billion tons.

The E.P.A. on Monday — announcing its official decision to reverse the rules — said that those standards were too expensive for automakers to meet and would ultimately hurt car owners by making vehicles costlier. The agency said that low oil prices meant savings at the pump would be less than forecast, and alluded to concerns that expensive fuel-saving technology could come at the cost of investments in auto safety.

The E.P.A. did not say how far the rules should be rolled back, only that it would start a new rule-making process to set “more appropriate” standards.

The Trump Administration’s action on Monday marked the latest in a series of tensions between California and the federal government. The E.P.A.’s aim to revise the climate change regulations were announced the same day the Justice Department moved to block a California law that regulates the transfer of federal land.

The Justice Department has also filed suit to overturn a California law intended to limit the power of the federal immigration authorities to act against people suspected of being illegal immigrants, and the federal government is seeking to make it easier to drill for oil and gas off the coast of California and other states. Furthermore, the tax overhaul law passed by Congress last year is projected to be particularly damaging for California, because it limits deductions for mortgage interest and state and local taxes, which are particularly high in the state.

Mr. Trump recently flew across the country to visit prototypes for a wall he would like to see built between California and Mexico, using the trip to describe California as “out of control.” And over the weekend, Mr. Trump attacked this state’s governor, Jerry Brown, in a tweet for pardoning five immigrants facing the threat of deportation.

But the fight over auto emissions standards is contentious and unprecedented, said Ann Carlson, a professor of environmental law at U.C.L.A. No administration, Democratic or Republican, has challenged California’s waiver under the Clean Air Act to set its own air quality rules.

“This is uncharted territory,” she said. “California has a long history of leading on auto emissions in a way that has been very productive for the country. California gets out in front and sees if regulations will work before the rest of the country adopts it,” she continued. “The administration is challenging that.”

Leading Democrats condemned the move. “As usual, the administration sides with big, powerful special interests over the interests of average American families, who will pay the price for lower miles per gallon and dirtier air,” said Chuck Schumer, the Senate Democratic Leader, in a statement.

In its defense of the Obama-era rules, California was joined Monday by a coalition of at least seven state attorneys general, including Eric Schneiderman of New York, as well as more than 30 mayors, who said they would “vigorously resist” any effort by the Trump administration to prevent states and cities from enforcing emissions standards. “We are committed to using our market power and our regulatory authority to ensure that the vehicle fleets deployed in our jurisdictions fully meet or exceed the promises made by the auto industry in 2012,” the coalition said.

If the Trump administration were to seek a permanent revocation of California’s authority to set its own rules, legal scholars say, it would require the Clean Air Act legislation to be amended, which only Congress could do. However, the Trump administration is more likely to revoke the more limited, specific waiver granted to California to set is own tailpipe emissions rules until 2025 — something it could try to settle in court.

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Scott Pruitt, the administrator of the Environmental Protection Agency, announced the plan in a news release.CreditTom Brenner/The New York Times

Automakers on Monday welcomed the E.P.A.’s move. The Association of Global Automakers, which represents the world’s biggest automakers, “appreciates” the agency’s decision that adjustments to the standards are needed, said John Bozzella, the industry group’s chief executive.

It is unclear how the industry would respond to a fractured American auto market if that were the result of the standoff with California. One possibility is that they sell substantially different car designs in the two markets. But they could also decide to adhere to the stricter California standards nationwide, muting the impact of the Trump administration’s rollback of federal rules.

There have been some signs of discord within the auto industry over the Trump administration’s plans.

Mr. Pruitt had been expected to publicly announce the effort on Tuesday at a Chevrolet dealership in suburban Virginia. But those plans were complicated by an angry pushback from some Chevy dealerships who were reluctant to see the brand associated with the announcement, according to two Chevy dealers who spoke on condition of anonymity, citing their relationship with General Motors. Late on Monday, the Virginia dealership, Pohanka Chevrolet in Chantilly, said the E.P.A. event it had planned to host had been canceled.

“They don’t want the E.P.A. to highlight Chevy,” said Adam Lee, chairman of Lee Auto Malls, which runs Nissan, Honda and Chrysler dealerships in Maine, and who said he was familiar with dealers’ thinking. “They don’t want to be the bad guys.”

“Trump has been saying these standards are crushing the auto industry. But we’ve had record years for the past four or five years, in terms of sales and profit,” he said. “It almost makes you think he doesn’t have the facts.”

A version of this article appears in print on , on Page A13 of the New York edition with the headline: California Vows to Fight U.S. Efforts to Reduce Car Pollution Standards. Order Reprints | Today’s Paper | Subscribe